1. Field of the Invention
An aspect of this disclosure relates to an image processing method, a nuclear medicine diagnosis apparatus, and a storage medium.
2. Description of the Related Art
Positron Emission Tomography (PET) is a technology for obtaining diagnostic images of the inside of a subject such as a human body or an animal. A PET apparatus can visualize the distribution of a radioactive isotope (RI) injected into the body of a subject.
In a diagnostic method using a PET apparatus, a test agent labeled (or marked) by a radioactive isotope that emits positrons is introduced into the body of a subject by, for example, injection or inhalation. The test agent introduced into the body is accumulated in a specific part of the body due to metabolism. The radioactive isotope labeling the test agent emits positrons, each emitted positron and a surrounding electron combine with each other and are thereby annihilated, and as a result, two gamma rays are emitted in opposite directions.
In a diagnosis method using a PET apparatus, the two gamma rays are detected and the detection results are processed by a computer to obtain distribution image data indicating the distribution of the radioactive isotope in the subject, and the subject is diagnosed based on a diagnostic image reconstructed from the distribution image data.
A gamma ray (radiation) detector for PET is implemented, for example, by a scintillation detector (e.g., bismuth germinate (BGO) or lutetium orthosilicate (LSO)). A scintillation detector can be manufactured at comparatively low costs. However, because a scintillation detector converts an incident gamma ray into light and converts the light into an electric signal using a photomultiplier tube, the resulting signal may be degraded during the conversion processes due to the spatial resolution and energy resolution.
On the other hand, a semiconductor detector can convert a gamma ray directly into an electric signal. For example, a semiconductor detector made of cadmium telluride (CdTe) has a higher spatial resolution and a higher energy resolution compared with a scintillation detector. However, a semiconductor detector made of cadmium telluride (CdTe) is less sensitive and more expensive than a scintillation detector.
Japanese Laid-Open Patent Publication No. 2008-232641, for example, discloses a method where Compton scattering is detected, and a gamma ray occurrence position detected by counting the coincidence at a pair of detectors (two-detector coincidence) is corrected based on the detected Compton scattering. Also, Japanese Laid-Open Patent Publication No. 2008-522168 discloses a method for reconstructing a cone from Compton scattering data by determining true events and accidental events. In a coincidence detection method, two or more sets of detection data obtained at a pair of detectors and having substantially the same gamma ray incident time are determined to be valid, sets of detection data having different gamma ray incident times are determined to be invalid, and image reconstruction is performed using the valid detection data to obtain a diagnostic image.
Here, to improve the sensitivity of a semiconductor detector, it is necessary to increase the volume of a semiconducting crystal such as a CdTe crystal. However, increasing the volume of a semiconducting crystal such as a CdTe crystal increases the costs of a semiconductor detector.
Also, the related-art methods disclosed in JP 2008-232641 and JP 2008-522168 are for correcting coincidence counting events. For example, in JP 2008-232641, the deviation of gamma rays from the 180 degree directions due to angular deviation is measured based on the principle of the Compton camera and an image is reconstructed taking into account the deviation; and in JP 2008-522168, a cone is reconstructed from Compton scattering data. Accordingly, the related-art methods do not increase the total number of valid events usable for reconstructing an image and therefore do not make it possible to obtain an image at high sensitivity.